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1、ZigBee: Wireless Technology for Low-Power Sensor NetworksGary Legg 5/6/2004 12:00 AM EDT Technologists have never had trouble coming up with potential applications for wireless sensors. In a home security system, for example, wireless sensors would be much easier to install than sensors that need wi
2、ring. The same is true in industrial environments, where wiring typically accounts for 80% of the cost of sensor installations. And then there are applications for sensors where wiring isnt practical or even possible.The problem, though, is that most wireless sensors use too much power, which means
3、that their batteries either have to be very large or get changed far too often. Add to that some skepticism about the reliability of sensor data thats sent through the air, and wireless sensors simply havent looked very appealing.A low-power wireless technology called ZigBee is rewriting the wireles
4、s sensor equation, however. A secure network technology that rides on top of the recently ratified IEEE 802.15.4 radio standard (Figure 1), ZigBee promises to put wireless sensors in everything from factory automation systems to home security systems to consumer electronics. In conjunction with 802.
5、15.4, ZigBee offers battery life of up to several years for common small batteries. ZigBee devices are also expected to be cheap, eventually selling for less than $3 per node by some estimates. With prices that low, they should be a natural fit even in household products like wireless light switches
6、, wireless thermostats, and smoke detectors.Figure 1: ZigBee adds network, security, and application-services layers to the PHY and MAC layers of the IEEE 811.15.4 radioAlthough no formal specification for ZigBee yet exists (approval by the ZigBee Alliance, a trade group, should come late this year)
7、, the outlook for ZigBee appears bright. Technology research firm In-Stat/MDR, in what it calls a cautious aggressive forecast, predicts that sales of 802.15.4 nodes and chipsets will increase from essentially zero today to 165 million units by 2010. Not all of these units will be coupled with ZigBe
8、e, but most probably will be. Research firm ON World predicts shipments of 465 million wireless sensor RF modules by 2010, with 77% of them being ZigBee-related.In a sense, ZigBees bright future is largely due to its low data rates20 kbps to 250 kbps, depending on the frequency band used (Figure 2)c
9、ompared to a nominal 1 Mbps for Bluetooth and 54 Mbps for Wi-Fis 802.11g technology. But ZigBee wont be sending email and large documents, as Wi-Fi does, or documents and audio, as Bluetooth does. For sending sensor readings, which are typically a few tens of bytes, high bandwidth isnt necessary, an
10、d ZigBees low bandwidth helps it fulfill its goals of low power, low cost, and robustness.Figure 2: ZigBees data rates range from 20 kbps to 250 kbps, depending on the frequency usedBecause of ZigBee applications low bandwidth requirements, a ZigBee node can sleep most of the time, thus saving batte
11、ry power, and then wake up, send data quickly, and go back to sleep. And, because ZigBee can transition from sleep mode to active mode in 15 msec or less, even a sleeping node can achieve suitably low latency. Someone flipping a ZigBee-enabled wireless light switch, for example, would not be aware o
12、f a wake-up delay before the light turns on. In contrast, wake-up delays for Bluetooth are typically around three seconds.A big part of ZigBees power savings come from the radio technology of 802.15.4, which itself was designed for low power. 802.15.4 uses DSSS (direct-sequence spread spectrum) tech
13、nology, for example, because the alternative FHSS (frequency-hopping spread spectrum) would have used too much power just in keeping its frequency hops synchronized.ZigBee nodes, using 802.15.4, can communicate in any of several different ways, however, and some ways use more power than others. Cons
14、equently, ZigBee users cant necessarily implement a sensor network any way they choose and still expect the multiple-year battery life that is ZigBees hallmark. In fact, some technologists who are planning very large networks of very small wireless sensors say that even ZigBee is too power hungry fo
15、r their uses.A ZigBee network node can consume extra power, for example, if it tries to keep its transmissions from overlapping with other nodes transmissions or with transmissions from other radio sources. The 802.15.4 radio used by ZigBee implements CSMA/CA (carrier sense multiple access collision
16、 avoidance) technology, and a ZigBee node that uses CSMA/CA is essentially taking a listen-before-talk approach to see if any radio traffic is already underway. But, as noted by Venkat Bahl, marketing vice president for sensor company Ember Corp. and vice chairman of the ZigBee Alliance, thats not a preferred approach. Having to listen burns power, says Bahl, and we dont like to do that.Another ZigBee and 802.15.4 communications o
